1. Field of the Invention
The present invention relates to an OFDM (Orthogonal Frequency Division Multiplexing) communication system, and more particularly to an apparatus and method for transmitting/receiving a preamble sequence having a minimum PAPR (Peak-to-Average Power Ratio) in an OFDM communication system using a plurality of transmission antennas.
2. Description of the Related Art
Typically, a wireless communication system provides its users or subscribers with wireless communication services, and is composed of a Node B and a UE (User Equipment). The Node B and the UE support the wireless communication services using a transmission frame. Therefore, the Node B and the UE must acquire mutual synchronization to transmit/receive the transmission frame. For this synchronization acquisition, the Node B transmits a synchronous signal, such that the UE can recognize a start point of the frame transferred from the Node B. The UE receives the synchronous signal from the Node B to recognize a frame timing of the Node B, and demodulates a reception frame according to the recognized frame timing. Generally, the synchronous signal is determined to be a specific preamble sequence engaged between the Node B and the UE.
The preamble sequence for use in the OFDM communication system uses a low PAPR (Peak-to-Average Power Ratio). A preamble transferred from the Node B to the UE is a long preamble, and is formed by connecting a preamble required for a coarse synchronization to a short preamble required for a fine frequency synchronization. A preamble transferred from the UE to the Node B acquires the fine frequency synchronization using only the short preamble. The reason why the low PAPR must be adapted as the preamble sequence for the OFDM communication system is as follows. The OFDM communication system uses a plurality of carriers (i.e. a plurality of sub-carriers) as a multi-carrier communication system, such that it highly considers orthogonality of the individual sub-carriers. Therefore, a phase is established between the sub-carriers, such that the mutual orthogonality is provided between the sub-carriers. If the phase is changed to another phase during a signal transmission/reception time through the sub-carriers, the signals between the sub-carriers may overlap one another. In this case, the magnitude of each signal overlapped with another signal due to the changed phase escapes from a linear interval of an amplifier included in the OFDM communication system, such that the signal transmission/reception cannot be normally performed, and thereby the OFDM communication system uses a preamble sequence having a minimum PAPR.
The OFDM communication system time-multiplexes one frame, such that it transmits data associated with a plurality of users (i.e. UEs). A frame preamble indicative of a frame start point in the OFDM communication system is transmitted during only a predetermined interval from the frame start point to a predetermined target point. Data may be irregularly transmitted to the respective UEs in the single frame, such that a burst preamble indicative of a data start point is positioned in front of each data. Therefore, the UE must receive the data preamble to recognize a transmission start point of the data. Specifically, the UE must establish synchronization with the data start point to receive the desired data. For this synchronization acquisition, the UE must acquire a preamble sequence commonly used in all the systems, and must establish synchronization with the acquired preamble sequence.
The OFDM communication system is the same as other communication systems, which do not use the OFDM scheme in various aspects, i.e. a source coding scheme, a channel coding scheme, and a modulation scheme, etc. The OFDM communication system performs an IFFT (Inverse Fast Fourier Transform) of data and inserts a guard interval into the IFFT-processed data, whereas a CDMA (Code Division Multiple Access) communication system spreads data and transmits the spread data. Therefore, the OFDM communication system can transmit a broadband signal using relatively simple hardware architecture as compared to the CDMA communication system. In more detail, if the OFDM communication system binds a plurality of bit/symbol sequences after modulating the data, and enters parallel bit/symbol sequences in the form of an IFFT entry signal corresponding to a frequency domain, a time domain signal in the form of an IFFT configuration is generated. In this case, the output time domain signal is configured by multiplexing a broadband signal with a plurality of narrow-band sub-carrier signals, and a plurality of modulation symbols are transmitted via the IFFT process during a single OFDM symbol interval.
However, if the OFDM communication system transmits the IFFT-processed OFDM symbol without any change, interference unavoidably occurs between a previous OFDM symbol and a current OFDM symbol. In order to remove the interference, the guard interval must be inserted. In this case, the guard interval is inserted using either one of a Cyclic Prefix scheme and a Cyclic Postfix scheme. The Cyclic Prefix scheme copies the last samples of the OFDM symbols contained in a time domain, and inserts the copied samples in an effective OFDM symbol. The Cyclic Postfix scheme copies the initial samples of the OFDM symbols contained in a time domain, and inserts the copied samples in an effective OFDM symbol. In this case, the samples for use in the Cyclic Prefix scheme and the Cyclic Postfix scheme are predetermined samples, and their magnitudes are also predetermined in the OFDM communication system. The guard interval may be adapted to acquire the time/frequency synchronization of an OFDM symbol received in a receiver using unique characteristics in which some parts of a single OFDM symbol of a time domain, i.e. the first part or the last part of the single OFDM symbol, are copied, and the copied parts are repeatedly arranged.
A transmission signal of a transmitter is distorted over an RF (Radio Frequency) channel, such that a receiver receives the distorted transmission signal. The receiver acquires the time/frequency synchronization of the distorted reception signal using a preamble sequence engaged between the transmitter and the receiver, performs a channel estimation of the distorted reception signal, and performs an FFT (Fast Fourier Transform) of the channel-estimation result, such that the distorted reception signal can be demodulated to frequency-domain symbols. After demodulating the frequency-domain symbols, the receiver applies a channel encoding scheme and a source decoding scheme to the demodulated symbols according to a channel coding method used in the transmitter, such that they are decoded into data.
The OFDM communication system uses a preamble sequence in three aspects, i.e. frame timing synchronization, frequency synchronization, and channel estimation. Needless to say, the OFDM communication system may also perform the frame timing synchronization, the frequency synchronization, and the channel estimation, etc., using a guard interval and a pilot sub-carrier, instead of using the preamble sequence. In the case of the preamble sequence, known symbols are transmitted at a forward part of each frame or data burst, and estimated time/frequency/channel information is updated at a data transmission part using various information, e.g., guard interval and pilot sub-carrier, etc.
The preamble sequence structure for use in the OFDM communication system will hereinafter be described with reference to FIGS. 1 and 2.
FIG. 1 is a diagram illustrating a long preamble sequence of a conventional OFDM communication system.
Prior to describing FIG. 1, a current OFDM communication system uses the same preamble sequence in the DL (Down Link) and the UL (Up Link) directions. Referring to FIG. 1, the long preamble sequence is configured by repeating a sequence of 64 lengths four times and repeating another sequence of 128 lengths two times. According to the aforementioned characteristics of the OFDM communication system, a Cyclic Prefix (CP) is positioned in front of a sequence in which the sequence of 64 lengths is repeated four times, and is also positioned in front of another sequence in which the sequence of 128 lengths is repeated two times. Also, the signals before performing the IFFT are frequency-domain signals, and the other signals after performing the IFFT are time-domain signals. The long preamble sequence shown in FIG. 1 is indicative of a long preamble sequence in a time domain after the IFFT has been performed.
In the meantime, a long preamble sequence in a frequency domain before performing the IFFT can be represented by the following expressions:
            S      ⁡              (                              -            100                    :          100                )              =                  {                                            +              1                        +            j                    ,          0          ,          0          ,          0          ,                                    +              1                        +            j                    ,          0          ,          0          ,          0          ,                                    +              1                        +            j                    ,          0          ,          0          ,          0          ,                                    +              1                        -            j                    ,          0          ,          0          ,          0          ,                                    -              1                        +            j                    ,          0          ,          0          ,          0          ,                                    +              1                        +            j                    ,          0          ,          0          ,          0          ,                                    +              1                        +            j                    ,          0          ,          0          ,          0          ,                                    +              1                        +            j                    ,          0          ,          0          ,          0          ,                                    +              1                        -            j                    ,          0          ,          0          ,          0          ,                                    -              1                        +            j                    ,          0          ,          0          ,          0          ,                                    +              1                        +            j                    ,          0          ,          0          ,          0          ,                                    +              1                        +            j                    ,          0          ,          0          ,          0          ,                                    +              1                        +            j                    ,          0          ,          0          ,          0          ,                                    +              1                        -            j                    ,          0          ,          0          ,          0          ,                                    -              1                        +            j                    ,          0          ,          0          ,          0          ,                                    +              1                        -            j                    ,          0          ,          0          ,          0          ,                                    +              1                        -            j                    ,          0          ,          0          ,          0          ,                                    +              1                        -            j                    ,          0          ,          0          ,          0          ,                                    -              1                        -            j                    ,          0          ,          0          ,          0          ,                                    +              1                        +            j                    ,          0          ,          0          ,          0          ,                                    -              1                        +            j                    ,          0          ,          0          ,          0          ,                                    -              1                        +            j                    ,          0          ,          0          ,          0          ,                                    -              1                        +            j                    ,          0          ,          0          ,          0          ,                                    +              1                        +            j                    ,          0          ,          0          ,          0          ,                                    -              1                        -            j                    ,          0          ,          0          ,          0          ,                                          ⁢          0          ,          0          ,          0          ,          0          ,                                    -              1                        -            j                    ,          0          ,          0          ,          0          ,                                    +              1                        -            j                    ,          0          ,          0          ,          0          ,                                    +              1                        +            j                    ,          0          ,          0          ,          0          ,                                    -              1                        -            j                    ,          0          ,          0          ,          0          ,                                    -              1                        +            j                    ,          0          ,          0          ,          0          ,                                    +              1                        -            j                    ,          0          ,          0          ,          0          ,                                    +              1                        +            j                    ,          0          ,          0          ,          0          ,                                    -              1                        +            j                    ,          0          ,          0          ,          0          ,                                    +              1                        -            j                    ,          0          ,          0          ,          0          ,                                    -              1                        -            j                    ,          0          ,          0          ,          0          ,                                    +              1                        +            j                    ,          0          ,          0          ,          0          ,                                    -              1                        +            j                    ,          0          ,          0          ,          0          ,                                    -              1                        -            j                    ,          0          ,          0          ,          0          ,                                    +              1                        +            j                    ,          0          ,          0          ,          0          ,                                    +              1                        -            j                    ,          0          ,          0          ,          0          ,                                    -              1                        -            j                    ,          0          ,          0          ,          0          ,                                    +              1                        -            j                    ,          0          ,          0          ,          0          ,                                    +              1                        +            j                    ,          0          ,          0          ,          0          ,                                    -              1                        -            j                    ,          0          ,          0          ,          0          ,                                    -              1                        +            j                    ,          0          ,          0          ,          0          ,                                    -              1                        +            j                    ,          0          ,          0          ,          0          ,                                    -              1                        -            j                    ,          0          ,          0          ,          0          ,                                    +              1                        -            j                    ,          0          ,          0          ,          0          ,                                    -              1                        +            j                    ,          0          ,          0          ,          0          ,                                    +              1                        +            j                          }            *              sqrt        ⁡                  (          2          )                    *              sqrt        ⁡                  (          2          )                                P      ⁡              (                              -            100                    :          100                )              =                  {                              -            1                    ,          0          ,                      +            1                    ,          0          ,                      +            1                    ,          0          ,                      +            1                    ,          0          ,                      +            1                    ,          0          ,                      -            1                    ,          0          ,                      -            1                    ,          0          ,                      +            1                    ,          0          ,                      -            1                    ,          0          ,                      +            1                    ,          0          ,                      -            1                    ,          0          ,                      -            1                    ,          0          ,                      +            1                    ,          0          ,                      +            1                    ,          0          ,                      -            1                    ,          0          ,                      +            1                    ,          0          ,                      -            1                    ,          0          ,                      +            1                    ,          0          ,                      -            1                    ,          0          ,                      +            1                    ,          0          ,                      -            1                    ,          0          ,                      +            1                    ,          0          ,                      +            1                    ,          0          ,                      -            1                    ,          0          ,                      +            1                    ,          0          ,                      -            1                    ,          0          ,                      -            1                    ,          0          ,                      +            1                    ,          0          ,                      -            1                    ,          0          ,                      -            1                    ,          0          ,                      -            1                    ,          0          ,                      +            1                    ,          0          ,                      +            1                    ,          0          ,                      -            1                    ,          0          ,                      +            1                    ,          0          ,                      +            1                    ,          0          ,                      +            1                    ,          0          ,                      -            1                    ,          0          ,                      +            1                    ,          0          ,                      +            1                    ,          0          ,                      -            1                    ,          0          ,                      -            1                    ,          0          ,                      -            1                    ,          0          ,                      +            1                    ,          0          ,                      +            1                    ,          0          ,                      +            1                    ,          0          ,                      +            1                    ,          0          ,                      +            1                    ,          0          ,                      +            1                    ,          0          ,                      +            1                    ,          0          ,                                          ⁢          0          ,                      0            ⁢                                                  -            1                    ,          0          ,                      -            1                    ,          0          ,                      +            1                    ,          0          ,                      -            1                    ,          0          ,                      -            1                    ,                      0            +            1                    ,                      0            +            1                    ,                      0            +            1                    ,                      0            -            1                    ,          0          ,                      +            1                    ,                      0            +            1                    ,          0          ,                      +            1                    ,          0          ,                      -            1                    ,          0          ,                      -            1                    ,          0          ,                      -            1                    ,          0          ,                      -            1                    ,          0          ,                      -            1                    ,          0          ,                      -            1                    ,          0          ,                      +            1                    ,          0          ,                      -            1                    ,          0          ,                      -            1                    ,          0          ,                      -            1                    ,          0          ,                      -            1                    ,          0          ,                      -            1                    ,          0          ,                      -            1                    ,          0          ,                      +            1                    ,          0          ,                      +            1                    ,          0          ,                      +            1                    ,          0          ,                      -            1                    ,          0          ,                      +            1                    ,          0          ,                      -            1                    ,          0          ,                      +            1                    ,          0          ,                      +            1                    ,          0          ,                      -            1                    ,          0          ,                      +            1                    ,          0          ,                      +            1                    ,          0          ,                      +            1                    ,          0          ,                      -            1                    ,          0          ,                      -            1                    ,          0          ,                      -            1                    ,          0          ,                      -            1                    ,          0          ,                      -            1                    ,          0          ,                      +            1                    ,          0          ,                      -            1                    ,          0          ,                      -            1                    ,          0          ,                      +            1                    ,          0          ,                      -            1                    ,          0          ,                      -            1                    ,          0          ,                      +            1                    ,          0          ,                      -            1                          }            *              sqrt        ⁡                  (          2          )                    *              sqrt        ⁡                  (          2          )                    
The long preamble sequences in the frequency domain, i.e. numerals shown in S(−100:100) and P(−100:100), are indicative of the positions of the sub-carriers used during the IFFT process, and will be described with reference to FIG. 3, such that its detailed description will herein be omitted for the convenience of description. S(−100:100) is indicative of a frequency-domain sequence in which a sequence of 64 lengths is repeated four times, and P(−100:100) is indicative of a frequency-domain sequence in which a sequence of 128 lengths is repeated two times. In the aforementioned S(−100:100) and P(−100:100), the reference symbol ‘sqrt(2)’ is indicative of the square root of 2, and the reference symbol ‘sqrt(2)*sqrt(2)’ is indicative of two-stage amplification to increase each Transmit Power of the S(−100:100) and P(−100:100).
The aforementioned description has disclosed the long preamble sequence with reference to FIG. 1, and a short preamble sequence will hereinafter be described with reference to FIG. 2.
FIG. 2 is a diagram illustrating a short preamble sequence of the conventional OFDM communication system.
Referring to FIG. 2, the short preamble sequence is configured by repeating a sequence of 128 lengths two times. According to characteristics of the OFDM communication system, a Cyclic Prefix (CP) is positioned in front of a sequence in which the sequence of 128 lengths is repeated two times. The short preamble sequence of FIG. 2 is indicative of a short preamble sequence in a time domain after the IFFT process has been performed. A short preamble sequence in a frequency domain is indicative of P(−100:100) shown in FIG. 2.
The aforementioned long preamble sequence must be generated satisfying the following four conditions:
1) First, the long preamble sequence must have a low PAPR.
To maximize the transmission efficiency of a PA (Power Amplifier) of a transmission end of an OFDM communication system's transmitter, a PAPR of an OFDM symbol must be low. In more detail, the IFFT-processed signal is applied to the PA, and a low PAPR is required due to the non-linear characteristics of the PA. The PAPR of the OFDM symbol must have a low ratio of maximum power to mean power of an OFDM time-domain symbol corresponding to an IFFT output end of the transmission end. In order to achieve the low ratio of the maximum power to the mean power, the PAPR of the OFDM symbol must achieve a uniform distribution. Specifically, if symbols each having a low cross-correlation are combined with each other in the frequency domain (i.e., an IFFT input end of the transmission end), an output PAPR is reduced.
2) Second, the long preamble sequence must be suitable for estimating parameters required for communication initiation.
The parameter estimation process includes a channel estimation process, a frequency offset estimation process, and a time offset estimation process.
3) Third, the long preamble sequence must include a low complexity and a low overhead.
4) Fourth, the long preamble sequence must be able to perform appropriate frequency offset estimation.
Various functions of the long preamble sequence generated considering the aforementioned items will be described as follows:
1) A sequence in which a sequence of 64 lengths is repeated four times is adapted to estimate a time offset and an appropriate frequency offset.
2) A sequence in which a sequence of 128 lengths is repeated two times is adapted to estimate a fine frequency offset and a channel.
In conclusion, the long preamble sequence is used for the following functions in the OFDM communication system.
1) The long preamble sequence is used as a first preamble sequence of a DL (Downlink) Protocol Data Unit (PDU).
2) The long preamble sequence is used in an initial ranging process.
3) The long preamble sequence is used in a bandwidth request ranging process.
The short preamble sequence is used for the following functions in the OFDM communication system.
1) The short preamble sequence is used as a UL (Uplink) data preamble sequence.
2) The short preamble sequence is used in a periodic ranging process.
The OFDM communication system can acquire the correct synchronization by performing the initial ranging process and the periodic ranging process, such that the UL data preamble sequence is mainly adapted to perform channel estimation. A variety of items are considered in the channel estimation process, for example, the PAPR, the performance, and the complexity. The PAPR for use in the conventional short preamble sequence is indicative of 3.5805 dB. A variety of channel estimation algorithms may be used as a channel estimation algorithm, for example, a Minimum Mean Square Error (MMSE) and a Least Square (LS), etc.
A relationship between the sub-carriers and the preamble sequence when the conventional OFDM communication system performs the IFFT process will hereinafter be described with reference to FIG. 3.
FIG. 3 is a diagram illustrating the relationship between the sub-carriers and the preamble sequence when the conventional OFDM communication system performs the IFFT process.
FIG. 3 assumes a predetermined condition in which the number of the overall sub-carriers of the OFDM communication system is 256, namely, there are 256 sub-carriers from −128-th sub-carrier to 127-th sub-carrier and the number of utilized sub-carriers is 200. Specifically, 200 sub-carriers (i.e. −100-th sub-carrier, . . . , −1st sub-carrier, 1st sub-carrier, . . . , 100-th sub-carrier) are used in FIG. 3. Input numbers positioned in front of an IFFT unit are indicative of the frequency components, i.e. the sub-carriers' number. In this case, only 200 sub-carriers from among 256 sub-carriers (i.e. 200 sub-carriers other than the 0-th sub-carrier, sub-carriers from −128-th sub-carrier to −101st sub-carrier, and other sub-carriers from 101st sub-carrier to 127-th sub-carrier, from among the 256 sub-carriers) are used. Null data (i.e. 0 data) is inserted into each of the 0-th sub-carrier, the sub-carriers from −128-th sub-carrier to −101st sub-carrier, and the sub-carriers from 101st sub-carrier to 127-th sub-carrier, and the resultant data having the null data is transmitted to a desired target for the following reasons. First, the reason why the null data is inserted in the 0-th sub-carrier is that a reference point of a preamble sequence in a time domain is indicative of a DC component in the time domain after the 0-th sub-carrier performs the IFFT process. Also, the reason why the null data is inserted into 28 sub-carriers from −128-th sub-carrier to −101st sub-carrier and 27 sub-carriers from 101st sub-carrier to 127-th sub-carrier is to provide a frequency domain with a guard interval because the above 28 sub-carriers and the above 27 sub-carriers correspond to a high frequency bandwidth in the frequency domain.
If the frequency-domain preamble sequence S(−100:100) or P(−100:100) is received in the IFFT unit, the IFFT unit maps the frequency-domain preamble sequence S(−100:100) or P(−100:100) to the corresponding sub-carriers in order to perform the IFFT process, such that a time-domain preamble sequence is generated.
A transmitter of the OFDM communication system will hereinafter be described with reference to FIG. 4.
FIG. 4 is a block diagram of a transmitter for use in the conventional OFDM communication system.
Referring to FIG. 4, if information bits to be transmitted are present, the information bits are received in a symbol mapper 411. The symbol mapper 411 modulates the information bits into symbols using a predetermined modulation scheme, and the symbols are transmitted to a Serial to Parallel (S/P) converter 413. In this case, the modulation scheme may be determined to be either a QPSK (Quadrature Phase Shift Keying) or a 16 QAM (Quadrature Amplitude Modulation), etc. The S/P converter 413 receives the output symbol of the symbol mapper 411, converts the received symbol into parallel data so that it is identical to an A point indicative of an input numeral of an IFFT (Inverse Fast Fourier Transform) unit 419, and outputs the parallel data to a selector 417. The preamble sequence generator generates a corresponding preamble sequence according to a control signal of a controller (not shown), and outputs the preamble sequence to the selector 417. The selector 417 selects either the output signal of the S/P converter 413 or output signals of the preamble sequence generator 415 according to a scheduling state of a corresponding time, and outputs the selected one to the IFFT unit 419.
The IFFT unit 419 receives the output signals of the selector 417, performs an A-point IFFT process on the received signals, and outputs the IFFT-processed result to a P/S (Parallel to Serial) converter 421. The P/S converter 421 receives a Cyclic Prefix of length ‘L’ along with the output signals of the IFFT unit 419. The P/S converter 421 converts the output signals of the IFFT unit 419 and the Cyclic Prefix into serial data, and outputs the serial data to a DAC (Digital to Analog Converter) 423. The DAC 423 receives the output signal of the P/S converter 421, converts the received signal into analog data, and outputs the analog data to an RF (Radio Frequency) processor 425. The RF processor 425 includes a filter and a front end unit, RF-processes the output signal of the DAC 423 so as to allow the output signal of the DAC 423 to be transmitted wirelessly, and transmits the RF-process result via an antenna.
A preamble sequence of the OFDM communication system using a plurality of transmission (Tx) antennas, e.g., N Tx antennas, and a method for generating the preamble sequence will hereinafter be described.
In the case of the OFDM communication system using the N Tx antennas, the preamble sequences must be transmitted over the N Tx antennas so as to estimate a channel of the data transmitted over each of the N Tx antennas passes. However, if the number of the Tx antennas of a current OFDM communication system is at least ‘2’, there is no method capable of transmitting the preamble sequences to estimate the channel of the data transmitted over each Tx antenna.
Provided that different sub-carriers are assigned to individual Tx antennas such that the assignment of the preamble sequences is established, a receiver is able to perform the channel estimation of the sub-carriers assigned for the preamble sequences, but is unable to perform the channel estimation of the remaining sub-carriers. Therefore, the preamble transmission regulations for performing the channel estimation of all of the sub-carriers are required for the individual Tx antennas on the condition that the number of the Tx antennas is at least ‘2’.
The OFDM communication system must use a preamble sequence having a low PAPR as previously stated above. Recently, many developers are actively conducting intensive research into a method for generating the preamble sequence having the low PAPR in a typical OFDM communication system, i.e. an OFDM communication system which uses a single Tx antenna. In the same manner as in the OFDM communication system using the single Tx antenna, another OFDM communication system using at least two Tx antennas (i.e. a plurality of Tx antennas) must use the preamble sequence having the low PAPR. However, a method for generating the preamble sequence having the low PAPR in the OFDM communication system which uses a plurality of Tx antennas has not been proposed yet, such that there is a need for the method for generating the preamble sequence having the low PAPR to be developed.
A channel estimation method in a frequency domain generally used in the OFDM communication system will hereinafter be described.
Provided that the number of input signals of the IFFT unit used in the OFDM communication system, i.e. the number of points of the IFFT unit, is ‘A’, and the number of real sub-carriers is ‘B’, a preamble sequence in a frequency domain can be represented by the following Equation 1:Xk, where k=−B/2, . . . −1, 1, . . . B/2  (1)
Provided that a channel response in the frequency domain is Hk, a signal generated when a receiver of the OFDM communication system FFT-processes its reception signal can be represented by the following Equation 2:Yk=HkXk+Zk, where k=−B/2, . . . −1, 1, . . . B/2  (2)
With reference to Equation 2, Zk is indicative of an AWGN (Additive White Gaussian Noise). In this case, a signal Yk generated when the FFT process is performed must be divided by a known signal Xk predetermined by mutual regulations between a transmitter and a receiver, and can be represented by the following Equation 3:H*k=Yk/Xk=Hk+Zk/Xk, where k=−B/2, . . . −1, 1, . . . B/2  (3)
Problems of the channel estimation operation of the OFDM communication system including a plurality of Tx antennas (e.g., N Tx antennas) will hereinafter be described.
First, the OFDM communication system including N Tx antennas must transmit preamble sequences over the N Tx antennas, such that it can correctly estimate a channel state of signals transmitted over each of the N Tx antennas. However, as stated above, a current OFDM communication system has no method capable of transmitting the preamble sequences using the N Tx antennas. Provided that the different sub-carriers are assigned to the individual N Tx antennas and then the preamble sequences are transmitted, although the receiver can perform the channel estimation of the sub-carriers assigned for transmitting the preamble sequences when the receiver performs a channel estimation process in the frequency domain, the receiver is unable to perform the channel estimation of the remaining sub-carriers.
Typically, the sub-carriers which are unable to perform the channel estimation may perform such channel estimation on the condition that the channel states of the channel-estimated sub-carriers are interpolated. As the frequency selective characteristic of the OFDM communication system is at a high level and the number of used Tx antennas increases, the channel estimation performance obtained by the above interpolation is deteriorated. Therefore, a channel estimation method capable of performing the channel estimation for all of the sub-carriers of the individual Tx antennas must be developed.